Whats that?

Ph issue.

Das sieht nach beginnenden Kalzium Mangel aus.

Dveloping calcium deficiency.

More often than not this is the start of a calcium deficiency. Make sure you are using a CalMag product during your grow. If you use RO water then likely you will need to use more because RO is nearly free of all calcium. Give the plant a good dose of CalMag for the next two waterings and it should reside and you will not see further rust looking spots develop. Hope this helps.

a ca deficiency or possibly too much K or P. Depends. this stuff is not discrete. more than one thing typically causes the symptoms you see. referencing how you've fed can help eliminate possibilities. even too little p can cause some copper-colered/dark dead spots... this is much easier to diagnose if in soilless/hydro context because you control all that the plant receives. Soil is a bit tougher to be certain due to unknown levels of nutes that came in the soil... as well as fertilizers that may or may not be plant-ready when added.. many need microbes to break them down before the plant can even use it, which isn't the case with soilless/hydro nutes.